add new W3C Manifest so Marketplace can be added to homescreens

read https://bugzilla.mozilla.org/show_bug.cgi?id=1084548#c0

I also added other tags to support other platforms' legacy "Add to Homescreen."
I was wondering why someone on iOS would install the marketplace app if they can't install anything in safari? But it's fine if they want to just browse or see what's going on. :+1: